The present invention relates generally to an improved communications system and in particular the present invention provides a method and apparatus for redirecting calls in a communications system. Still more particularly, the present invention provides a method and apparatus for redirecting calls in an overloaded cell in a wireless communications system.
The recent surge in the popularity of cellular radio and mobile service in general has resulted in an overall increase in traffic and a shortage of available system capacity in large metropolitan areas. Current cellular systems exhibit a wide range of traffic densities, from low in rural areas to overloading in downtown areas, with large daily variations between peak hours and quiet night hours. It is a system engineering challenge to design a communications system that will make optimal use of the available frequency spectrum and offer a maximal traffic throughput (e.g. erlangs/megahertz/service area) at an acceptable service quality. In a cellular environment, the overall system capacity in a given service area is a product of many (complexly interrelated) factors, such as, the available frequency spectra, service quality, traffic statistics, type of traffic, type of protocol, shape and size of service area, selected antennas, diversity, frequency reuse capability, spectral efficiency of coding and modulation schemes, and efficiency of multiple access.
One problem encountered in wireless communications systems is the problem associated with an xe2x80x9coverloadxe2x80x9d condition in a cell. An xe2x80x9coverloadxe2x80x9d condition is defined as a cell that is in-a state in which all voice channel resources allocated for call setup are currently in use. When a cell is in an overload condition, all access attempts to this cell will beyond this point either fail or be re-directed by the communications system to an adjacent cell, depending on how the communications system is configured. If the communications system is configured for xe2x80x9cdirected retryxe2x80x9d, the communications system will re-direct mobile accesses once a certain voice channel resource threshold is reached.
With directed retry in older cellular systems, a mobile station attempts to access the communications system. The communications system receives the access attempt. If the communications system determines that the cell on which the access attempt has occurred is in an overload condition, the communications system sends a directed retry message to the mobile station. In this example, the message is sent with 1 to 6 possible adjacent cell analog control channel (ACCH) offsets. The mobile station receives this directed retry message, attempts to find the best possible ACCH based on the channel offsets received, and re-attempts the access on the best ACCH identified.
One limitation with this type of mechanism that the communications system is unaware of is the best adjacent ACCH to which the call should be redirected. Since the communications system can only send down a list of ACCH offsets to the mobile station based on system configured adjacent cell information and has no idea as to which one the mobile station will select, the communications system also cannot specify feature specification information (i.e., authentication) to the mobile station because the communications system is not sure on which cell the mobile station will re-access.
Directed retry on the newer digital control channel (DCCH) in a communications system under Interim Standard (IS) 136 has limitations as well. IS 136 was developed by the Cellular Telecommunications Industry Association (CTIA). With IS 136 and DCCHs, directed retry was enhanced to support two methods of operation. The first method mimics that of directed retry on an ACCH. Once the system determines that the cell is in an overload condition and a mobile station access is attempted on that cell, the communications system can send a directed retry message down to the mobile station with channel specific information for the mobile station to use to re-access an adjacent cell. The second method of directed retry on a DCCH in an IS 136 system allows the communications system to just send a directed retry to the mobile station without any channel specific information. The mobile station can then use neighbor cell list information being broadcast on the serving DCCH to determine which adjacent cells are marked as directed retry cells. The mobile station then executes reselection procedures as defined in IS 136 to determine the next best ACCH or DCCH on which to re-attempt the access. The mobile station only attempts reselection to the adjacent cells that have been marked as directed retry cells in the neighbor cell message. Both of these methods of re-directing a mobile station""s access due to cell overload have the same limitations as on ACCHs. For example, the communications system has no idea where the mobile station will re-attempt the access. Since the communications system has no idea on which cell the mobile station will re-attempt the access, the communications system cannot inform the mobile station of specific features available on the cell to which the mobile station is beign re-directedxe2x80x94read control filler (RCF) and authentication (AUTH) if being re-directed to any ACCH. Further, the mobile station may attempt to re-access on an adjacent cell, which is also in an overload state. This situation would cause unnecessary messaging on the communications system. Moreover, processing of the directed retry secnario may not be carried out exactly the same by different model mobile stations. This situation leads to difference in how mobile stations are being redirected within the same cell.
Therefore, it would be advantageous to have an improved method and apparatus for redirecting mobile stations attempting access to cell in an overload condition.
The present invention solves problems associated with redirecting a call by a mobile station accessing an overloaded cell in a wireless communications system. The mechanism of the present invention uses mobile assisted channel allocation information to identify an adjacent channel for use in redirecting a mobile station. The communications system monitors for attempts by the mobile station to access a first cell in an overloaded state. While in an idle state, the mobile station may monitor and store signal strengths in cells adjacent to the first cell in which a mobile station is located. When such an access is detected, information is obtained from the mobile station for cells adjacent to the first cell. A second cell is identified from the cells adjacent to the first cell using the information, and a message is sent to the mobile station to access the second cell.
The information sent to the communications system by the mobile station includes signal strengths for the adjacent cells. The information is sent in a report that may be initiated by the communications system. The second cell may be selected by prioritizing the adjacent cells. The ranking or prioritizing of the cells is made from adjacent cells that are not in an overload state and may be based on the signal strengths returned by the mobile station. Other factors also may be used to rank or prioritize the adjacent cells, such as, for example, whether the voice channel resources match those requested by the mobile station and whether features in first cell are found in adjacent cells, and whether the system determines that the cell will have sufficient quality to set up on an adjacent cell.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.